Superconductivity is a phenomenon characterized by zero electrical resistance occurring in certain materials below a characteristic temperature. It was discovered by Heike Kamerlingh-Onnes on Apr. 8, 1911 in Leiden. Like ferromagnetism and atomic spectral lines, superconductivity is a quantum mechanical phenomenon. It is further characterized by the Meissner effect, the complete ejection of magnetic field lines from the interior of the superconductor as it transitions into the superconducting state. The occurrence of the Meissner effect indicates that superconductivity cannot be understood simply as the idealization of perfect conductivity in classical physics.
The electrical resistivity of a metallic conductor decreases gradually as temperature is lowered. In ordinary conductors, such as copper or silver, this decrease is limited by impurities and other defects. Even near absolute zero, a real sample of a normal conductor shows some resistance. In a superconductor, the resistance drops abruptly to zero when the material is cooled below its critical temperature. An electric current flowing in a loop of superconducting wire can persist essentially indefinitely with no additional power source.
High-temperature superconductors are materials that have a superconducting transition temperature (Tc) above 30 K (−243.2° C.). From 1960 to 1980, 30 K was thought to be the highest theoretically possible Tc. The first high-Tc superconductor was discovered in 1986 by IBM researchers Karl Müller and Johannes Bednorz, for which they were awarded the Nobel Prize in Physics in 1987.